The stability constants of the 1:1 complexes formed between Mg2+, Ca2+, Ba2+, Mn2+, Co2+, Ni2+, Zn2+, or Cd2+ and AMPS2-, i.e., of the M(AMPS) complexes, were determined by potentiometric pH titrations (25 °C; I = 0.1 M, NaNO3). For the Mn2+/AMPS, Co2+/AMPS, Ni2+/AMPS, and Cd2+/AMPS systems also the protonated species M(H;AMPS)+ were quantified, and for the Zn2+/AMPS system, the stability of the hydroxo species Zn(AMPS)(OH)-, which results from the Zn2+−thio coordination, could be determined. On the basis of previously established log versus p straight-line plots (R-MP2- = simple monophosphate ester ligands without further coordinating groups; Sigel, H.; et al. Helv. Chim. Acta 1992, 75, 2634), it is concluded that the alkaline earth ions in the M(AMPS) complexes are coordinated to the thiophosphate group with the same intensity as to a normal phosphate group. For the M(AMPS) complexes of Mn2+, Co2+, Ni2+, Zn2+, and Cd2+, it is shown by comparison with the corresponding M(AMP) complexes and by employing t...